The achievement of satisfactory combustion of the fuel in a gas turbine engine has always presented problems. As a minimum requirement it is essential for the fuel to be atomized into a spray of small drops at all operating conditions and to obtain this result over the wide range of fuel flows necessary (typically maximum/minimum = 100) has required the development of complex and sophisticated fuel spray nozzles. It is well known to use swirl-atomizers in which the fuel is supplied at high pressure to a swirl-chamber in which a free vortex is formed so that the fuel issues from the discharge orifice of the swirl-chamber as a thin sheet of conical shape which breaks up into a spray of drops by interaction with the surrounding air; these are known conventionally as "pressure atomizers". Since a pressure atomizer can only produce a satisfactory spray over a flow range of about 7:1 (maximum:minimum) it has been necessary to combine two pressure atomizers, one of low flow capacity known as a "pilot" or "primary" and the other of high flow capacity, known as a "secondary", into a single fuel nozzle such as is disclosed in U.S. Pat. No. 3,013,732 which is conventionally known as a "dual orifice" nozzle.
To obtain improved atomization compared with the pressure atomizer it is well known to use high velocity and/or high pressure air as the means of atomizing the fuel, as disclosed in U.S. Pat. No. 3,474,970 and No. 3,283,502. In the former the air is supplied from a source external to the engine and the nozzle is known as an "air-assisted" type. In the latter the air is available inside the engine and this is known as an "air-blast" type. Although the fuel flow range for satisfactory atomization of both "air-assist" and "air-blast" types is greater than a single "pressure atomizer" there are many applications in which it is considered necessary or desirable to combine an air-atomizing nozzle with a pressure atomizer as is disclosed in U.S. Pat. No. 3,912,164. In such an arrangement the pressure atomizer is used for the low fuel flow rate conditions, such as starting the engine, while the air-atomizer is used for the higher fuel flow rates, and this combination is usually described as a "hybrid" type.
With both the dual-orifice and hybrid types of nozzle it is the invariable practice to maintain the "pilot" or "primary" nozzle flowing at all times so that at the higher fuel flows the primary and secondary atomizers are both in operation. There are some disadvantages in this arrangement since the shape of the primary spray is often different from that of the secondary spray and can result in a non-optimum placement of fuel in the combustion chamber. For example, if the primary spray angle is less than the secondary (which may be desirable to obtain good starting) then at high power conditions when the secondary also is in operation, the primary fuel may be concentrated in the center of the total spray and this produces smoke in the engine exhaust. The obvious solution to this problem is to shut off the primary nozzle fuel flow at high power conditions but this has been found to be impractical since the residue of fuel left in the primary nozzle readily carbonizes at the high metal temperatures prevalent at these operating conditions and the primary nozzle fuel flow passages can become plugged with carbon. A compromise solution is to reduce the primary fuel flow after the secondary fuel flow has reached a certain value, as disclosed in U.S. Pat. No. 3,675,853, but this requires the use of additional valve means located in the hottest operating environment, which is not conducive to the high reliability of operation demanded.
Ideally, therefore, what is needed is a fuel nozzle, having all the known advantages of an air-atomizer and also the wide flow range capability of a hybrid design, in which the spray from the primary ceases to exist as a separate entity when the secondary is in operation.